The manufacture of flat forms from refractory metal powder often includes compaction, such as by rolling, of the metal powder to a green compact, followed by liquid phase sintering of the compacted powder. Here and throughout this application the term "compaction" means the compression of a mass of metal powder to an abiding form e.g., a green compact. Whereas the term "consolidation" means the densification of a mass of metal powder whether by compaction or other means, such as sintering or mechanical hot working. Further consolidation such as by mechanical hot working after sintering is usually required to obtain the desired net shape. Known processes for producing such flat forms as plate have several disadvantages.
First, roll compaction of refractory metal powders is difficult because the fine, brittle particles flow poorly and do not readily weld under pressure during rolling. Ready et al., U.S. Pat. No. 3,245,114 issued Apr. 12, 1966 relates to apparatus which is claimed to provide "satisfactory and reproducible rolling" of powders of tungsten and tungsten alloys. Ready et al. describes a complex roll compacting apparatus, but it is not disclosed or suggested that the apparatus is capable of rolling strip much wider than 4 inches, e.g., 12 inch wide strip. The Ready et al. patent, however, does not disclose what effect the powder characteristics have on roll compactibility. In a related publication, T. J. Ready and H. D. Lewis, "A Technique for Powder Rolling Tungsten and Tungsten--45 v/o UO.sub.2 Dispersions", Int'l J. of Powder Met. Vol. 1, No. 2 (1965), it was suggested that roll compactibility of a powder was primarily dependent on particle size distribution and particulate characteristics.
Another disadvantage of known powder roll compaction processes is the difficulty in consistently forming green compacted flat forms with dry powder. P. C. Eloff, "The Technology of Powder Rolling", Carbide and Tool Jour. (May/June 1986) and Grab et al., U.S. Pat. No. 4,491,559 describe the use of binders to overcome some of the problems associated with roll compaction of dry powders. However, the use of binders requires an extra processing step to remove the binder material prior to sintering. Nevertheless, all of the binder material cannot be removed resulting in the retention of undesirable impurities in the finished product.
Another disadvantage of the known methods of producing flat forms from metal powder is the limitation on thickness which can be practically obtained. Thick plate manufactured by roll compaction is not practical because extremely large diameter rolls would be required.
A further disadvantage of the known processes is that catastrophic blistering can occur during liquid phase sintering. Such blistering renders the flat form product unusable. Although the mechanism by which such blistering occurs has been studied, there has been no effective method for eliminating or at least significantly controlling such blistering.
A still further disadvantage of the known processes is the retention in the sintered material of undesirable impurities such as hydrogen and oxygen which can result in embrittlement and poor mechanical properties. Special sintering cycles have been devised, such as that described in Bose et al., "Liquid Phase Sintering of Tungsten Heavy Alloys in Vacuum," Proc. of the Annual Powder Metallurgy Conf. and Exhibit, May, 1986, in an attempt to overcome impurity entrapment. However, such special sintering cycles require a post-sintering heat treatment to remove residual hydrogen and obtain the desired properties. Such additional post-sintering processing results in a substantial economic disadvantage.